Long-Term Regional Reanalysis for Japan with Assimilating Conventional Observations (RRJ-Conv)

Abstract

We are conducting a 5-km long-term atmospheric regional reanalysis for Japan with assimilating conventional observations (RRJ-Conv). RRJ-Conv is produced with a one-way double-nesting system consisting of a nonhydrostatic regional model and a local ensemble transform Kalman filter (LETKF), which is driven by the Japanese 55-year reanalysis (JRA-55). The assimilated data are limited to long-term available data, specifically surface in-situ pressure observations, upper-air radiosonde observations, and tropical cyclone center positions. This paper overviews the performance of RRJ-Conv for 20 years from July 2001 to June 2021, mainly focusing on precipitation and exploring added values to JRA-55. RRJ-Conv is confirmed to maintain long-term consistency of analysis quality. Compared to JRA-55, RRJ-Conv reduces biases in central pressures of tropical cyclones, maintaining position reproducibility. RRJ-Conv represents detailed spatial distributions of monthly precipitation, extreme values for daily precipitation, and their interannual variation more realistically than JRA-55. The improvements to JRA-55 are demonstrated for some extreme events, involving a tropical cyclone, Baiu front and East Asian winter monsoon.